The present invention relates to a method and apparatus for driving an electric vehicle having an inverter for converting DC electric power (such as from a battery) into AC electric power, and a synchronous motor as the driving force of the vehicle, particularly a permanent-magnet type synchronous motor.
Development of a permanent-magnet type motor (hereinafter sometimes referred to simply as a "synchronous motor") as a motor for driving an electric vehicle is progressing. The efficiency of the permanent-magnet type synchronous motor can be increased in a low speed and low torque region compared to an induction motor. An electric vehicle driving system using such a permanent-magnet type synchronous motor is disclosed, for example, in Japanese Patent Application Laid-Open No. 6-315201.
In an electric vehicle, for example an electric automobile, in order to obtain a braking force to the vehicle, so-called regenerative braking may be performed using the motor as a generator. In such an electric vehicle, there are some cases in which the motor may be driven at a high speed, above a maximum controllable rotating speed, during coasting on a descending road.
In controlling synchronous motors generally used in the industrial field, when the motor speed increases to a value which exceeds the controllable range, the motor is judged to be in an abnormal condition. In the case of an electric vehicle, the control system is designed so that the motor is controlled by repeatedly driving it up to a maximum allowable speed for the power-train system, and then operating it in a normal re-operation within the controllable range as the speed of the motor is decreased.
When the motor is driven at a speed above the maximum controllable speed, a protective relay provided between the inverter and the motor is actuated, so that the inverter is disconnected to protect it, (and also the battery and control circuits) against a high voltage induced in the permanent-magnet type synchronous motor (generator) when it is driven at a speed above the maximum controllable speed.
However, provision of an additional protective relay between the inverter and the motor makes control complex, the system large in size and higher in cost.